In the field of the interpretation of seismic data obtained for purposes of oil and gas exploration, one of the more important parameters to be determined is propagation velocity of acoustic energy through sub-surface rock. One may identify the lithology of a sub-surface layer by knowledge of its interval velocity, where interval velocity refers to the acoustic propagation velocity within a given sub-surface layer, region or stratum. Furthermore, knowledge of the interval velocity for a hydrocarbon bearing region can assist in predicting the geopressure therein.
As is well known in the art, the interval velocities of sub-surface strata are essential in determining the depths of acoustic reflectors from the two-way times of acoustic waves generated and sensed in conventional seismic prospecting. Accordingly, the accuracy with which interval velocities may be determined within a survey volume will determine the accuracy with which the depths of the sub-surface geological features can be determined in the survey.
Data from which interval velocities are determined can be provided by several conventional sources. Actual velocities for particular sample locations may be obtained by core sampling or by conventional well logging. While the velocity information obtained by sampling and logging is quite accurate, the volume over which the velocity information is valid is necessarily quite small, however.
Modern seismic survey data processing techniques are now also used to determine certain velocity parameters that are often referred to as "process" velocities, from which interval velocities may be deduced. One process velocity is the so-called "stack" velocity that may be derived from 2-D seismic data using the well-known techniques of constant velocity gather ("CGP") and constant velocity stack ("CVS"). The CGP and CVS processes each perform normal move-out ("NMO") for a set of 2-D seismic data over iterations of an assumed constant velocity; the closest stack velocity to the actual velocity is presumed to be that which results in the most realistic reflection artifact. Another well-known process velocity is the dip move-out ("DMO") velocity, which is used to correct for the angle of dip of a sub-surface reflector. Conventional migration techniques also produce a migration velocity, which is another type of process velocity, which is based upon the assumed location of equal velocity lines below the surface and above the reflector being migrated.
Each of these process velocities, as well as those obtained from core sampling and logging, are used to estimate interval velocities for a number of surface locations using the well-known Dix equation. While this approach results in a calculated interval velocity, both the process velocity values and the Dix equation itself are based upon assumptions about the sub-surface geology. These assumptions may not be valid for the actual survey volume, however, due to differences between the assumptions and the actual geology which manifest themselves as anomalies or artifacts in the interval velocity model for the survey. Examples of physical causes of these artifacts include those well-known problems due to near-surface layers, such as permafrost; near-surface layers of high velocity and significant thickness variation can especially causes problems in the stacking of seismic data.
Furthermore, considering that process velocity values are generally optimized to provide the best image of a reflector in the seismic data rather than the most accurate representation of the actual acoustic velocity in the earth, use of process velocities in performing time-to-depth conversion of seismic data is vulnerable to error.
According to prior data processing techniques, anomalies and artifacts in the calculated interval velocity volume have been dealt with by numerically smoothing the measured velocities prior to their conversion to interval velocities by way of the Dix equation. It has been observed, however, that the resulting interval velocity volume remains inaccurate relative to the actual geology, however, as this prior method merely distributes (or "smears out") the anomalies over a larger volume rather than removing them from the model.
By way of further background, an approach for performing time-to-depth conversion using seismic velocities is described in Carter, et al., "North Sea velocity correction techniques", Oil & Gas Journal (Oct. 24, 1988), pp. 81-84. According to this approach, velocities are manually repicked for selected horizons. However, this repicking is extremely cumbersome and requires a large amount of manual labor. Furthermore, as described in the article, these repicked velocities are numerically smoothed, and accordingly any velocity anomalies that may be present are not removed.
Prior data processing techniques have also included the use of supercomputers to iteratively perform depth migration, by way of an inversion, over a survey volume based on multiple 2-D survey lines. It has been observed, however, that this iterative inversion process not only requires large amounts of supercomputer time (with costs of on the order of tens of thousands of dollars of computer time for each survey line), but also still requires a great deal of interpretation by a human expert between inversions to arrive at a usable result.
By way of further background, the GEOQUEST, LANDMARK and CHARISMA seismic data interpretation systems are known to provide graphical display of cross-sections of seismic data with common-depth point (CDP) on the horizontal axis and two-way time on the vertical axis. In such cross-section displays, these systems "post" two-way time information from seismic lines that cross the displayed line.
It is therefore an object of the present invention to provide a method of determining a velocity model for a survey volume which more accurately utilizes velocity data from multiple sources.
It is a further object of the present invention to provide such a method where the modeled velocities may be interval velocity or may be root-mean-square velocity.
It is a further object of the present invention to provide such a method which operates in an interactive manner with a human analyst.
It is a further object of the present invention to provide such a method which allows the human analyst to locate and remove anomalous information from the interval velocity survey volume.
It is a further object of the present invention to provide such a method which allows the human analyst to incorporate velocity data from dissimilar sources, such as well logs, core samples and the like into the process velocity model to arrive at the interval velocity volume.
It is a further object of the present invention to provide such a method which can receive processing velocities from a number of conventional sources.
It is a further object of the present invention to provide such a method which can present its output to conventional time-to-depth conversion programs.
It is a further object of the present invention to provide such a method having the capability of subsequent updating of velocity data or new seismic lines to provide an upgraded velocity volume.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.